[en] Elastic strain is potentially an important approach in tuning the properties of the improperly multiferroic hexagonal ferrites, the details of which have however been elusive due to the experimental difficulties. Employing the method of restrained thermal expansion, we have studied the effect of isothermal biaxial strain in the basal plane of h-LuFeO_3 (001) films. The results indicate that a compressive biaxial strain significantly enhances the K_3 structural distortion (the order parameter of the improper ferroelectricity), and the effect is larger at higher temperatures. The compressive biaxial strain and the enhanced K_3 structural distortion together, cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in h-LuFeO_3, according to our first principle calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in h-LuFeO_3. Finally, the experimental elucidation of the strain effect in h-LuFeO_3 films also suggests that the restrained thermal expansion can be a viable method to unravel the strain effect in many other thin film materials.

[en] The surface termination and the nominal valence states for hexagonal LuFeO_3 thin films grown on Al_2O_3(0 0 0 1) substrates were characterized by angle resolved x-ray photoemission spectroscopy. The Lu 4f, Fe 2p and O 1s core level spectra indicate that both the surface termination and the nominal valence depend on surface preparation, but the stable surface terminates in a Fe–O layer. This is consistent with the results of density functional calculations which predict that the Fe–O termination of LuFeO_3(0 0 0 1) surface is energetically favorable and stable over a broad range of temperatures and oxygen partial pressures when it is reconstructed to eliminate surface polarity. (paper)

[en] The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H₂B(pz)₂}₂(bipy)] polycrystalline films on Al₂O₃ substrates have been studied, where pz  =  pyrazol-1-yl and bipy  =  2,2′-bipyridine. For [Fe{H₂B(pz)₂}₂(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H₂B(pz)₂}₂(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H₂B(pz)₂}₂(bipy)]/Al₂O₃ interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H₂B(pz)₂}₂(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition. (paper)

[en] The electronic structure for the conduction bands of both hexagonal and orthorhombic LuFeO_3 thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectral features and the linear dichroism are observed. These differences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states have not changed, the spectra are sensitive to the changes in the local environments of the Fe"3"+ and Lu"3"+ sites in the hexagonal and orthorhombic structures. Using the crystal-field splitting and the hybridizations that are extracted from the measured electronic structures and the structural distortion information, we derived the occupancies of the spin minority states in Fe"3"+, which are non-zero and uneven. The single ion anisotropy on Fe"3"+ sites is found to originate from these uneven occupancies of the spin minority states via spin–orbit coupling in LuFeO_3. (paper)

[en] Ferroelectricity at room temperature has been demonstrated in nanometer-thin quasi 2D croconic acid thin films, by the polarization hysteresis loop measurements in macroscopic capacitor geometry, along with observation and manipulation of the nanoscale domain structure by piezoresponse force microscopy. The fabrication of continuous thin films of the hydrogen-bonded croconic acid was achieved by the suppression of the thermal decomposition using low evaporation temperatures in high vacuum, combined with growth conditions far from thermal equilibrium. For nominal coverages ≥20 nm, quasi 2D and polycrystalline films, with an average grain size of 50–100 nm and 3.5 nm roughness, can be obtained. Spontaneous ferroelectric domain structures of the thin films have been observed and appear to correlate with the grain patterns. The application of this solvent-free growth protocol may be a key to the development of flexible organic ferroelectric thin films for electronic applications.